The present invention relates to a method of calibrating a thickness measuring device. The present invention relates in particular to a method of calibrating a thickness measuring device with two noncontacting or scanning displacement measuring sensors.
Furthermore, the present invention relates to a device for measuring or monitoring the thickness of layers, tapes, foils, and the like, having at least two noncontacting or scanning displacement measuring sensors arranged side by side or opposite to one another, in particular for carrying out the method of the invention.
Thickness measuring devices in the most different designs are known from practice, and operate either by the noncontacting or scanning method.
When measuring or monitoring the thickness of tapes, the advancing tapes are normally measured in their thickness by two oppositely arranged displacement measuring sensors. In so doing, and as illustrated in FIG. 1, the thickness D is determined from the distances A, B which the sensors have relative to the tape to be monitored, and from the distance C of the displacement measuring sensors relative to one another. The computing operation to determine the tape thickness D is: EQU D=C-(A+B).
Depending on the material and speed of the tape, the environment, and required accuracy, different measuring principles are to be applied when measuring a thickness, i.e., displacement measuring sensors operating by different measuring principles.
When it comes to monitor metal tapes with respect to their thickness, it is possible to use, at a low tape speed and with an unsensitive tape surface, inductive measuring scanners with a rolling or sliding tracer tip. With a sensitive tape surface, and/or at a high speed of the tape, and/or an action of force upon the tape which is to be avoided, noncontacting thickness measuring devices with eddy-current sensors, capacitive or optical sensors are used, which do not cause wear on the tape.
However, when nonmetallic tapes are to be monitored as regards their thickness, the thickness measuring device will then operate with inductive measuring scanners, when the tape is transported at a low speed, or when it has an unsensitive surface. In the case of a sensitive tape surface, soft tape material, clean environment, high tape speed, and undesired action of force on the tape, the measuring is mostly noncontacting with capacitive or optical sensors. Within the scope of noncontacting distance measuring, it is also possible to use an eddy-current sensor floating at a constant distance above the tape.
Naturally, the foregoing description also applies to the thickness measurement on foils, and to measurements of layer thicknesses of insulating and conducting materials.
However, the displacement measuring sensors used in the known thickness measuring devices are problematic in practice, since they exhibit a nonlinear behavior within their measuring range. As a result of a fluttering of the tapes to be monitored, the different thicknesses of the tape, or the movement of the tape toward or away from the displacement measuring sensors, the latter measure different distances. These different distances are subjected to errors in accordance with the nonlinear behavior of the displacement measuring sensors over the measuring range, so that finally the thickness resulting from the above discussed equation, i.e. from the individual distances measured by the displacement measuring sensors, is likewise subjected to errors.
Until now, it has been attempted to determine the nonlinearity of the individual displacement measuring sensors by "traversing" the entire measuring range of each displacement measuring sensor by means of an object guided by a micrometer screw, and to thus calibrate the displacement measuring sensors. Such a calibration is done, because of the apparatus situation, either in the lab of the instrument manufacturer or at the location by the expert personnel of the instrument manufacturer. Measuring errors caused at the measuring location by temperature fluctuations or thermal displacements, long-term drifts of the displacement measuring sensors, or the like are not considered in conventional thickness measuring devices. Rather, a costly subsequent calibration of the known thickness measuring device is required which, in consideration of the environment, must be carried out by the service personnel of the instrument manufacturer at the location, or under simulated environmental conditions in the lab of the manufacturer.